High resolution non-impact printer

ABSTRACT

An apparatus and method for high resolution non-impact printing. A pulsed electric field is applied between a shaping and base electrode through a donor sheet and a closely adjacent recipient sheet to transfer electrically conductive printing material particles from the donor sheet to the recipient sheet. The shape of the resultant deposit of printing material particles on the recipient sheet is related to the spatial distribution of the pulsed electric field components passing through the donor and recipient sheets. The distribution of those components is primarily controlled by the shape of the printing face of the shaping electrode. A shield electrode is disposed about the outer contour of the printing face of the shaping electrode and extends along the side surfaces of that electrode for cancelling the effect of fringe components of the pulsed electric field emanating from points on the surface of the shaping electrode outside the printing face.

United States Patent 11 1 Koch 1 1 HIGH RESOLUTION NON-IMPACT PRINTER [75] Inventor: Paul L. Koch, Saugus, Mass.

[73] Assignee: The Carters Ink Company,

Cambridge, Mass.

[22] Filed: Aug. 10, 1973 [21] Appl. No.: 387,347

52 Us. Cl 346/74 EH; 346/74 R [51] Int. Cl. G0ld 15/06 [58] Field of Search 346/74 E. 74 ES, 74 SC. 346/74 CH, 74; 346/139 C [56] References Cited UNITED STATES PATENTS 2,830 867 4/1958 Kohn et a1 346/74 CH 3,358,289 12/1967 Polee v .1 346/74 ES 3,438,053 4/1969 Howell t 346/74 ES 3550,153 12/1970 Haeberle et al..... 346/74 E 3.733613 5/1973 Koch et al 346/74 E Aug. 5, 1975 Primary Examiner-Bernard Konick Assistant E.ranziner-Jay P. Lucas Attorney, Agent. or FirmKenway & Jenney [5 7] ABSTRACT An apparatus and method for high resolution nonimpact printing. A pulsed electric field is applied between a shaping and base electrode through a donor sheet and a closely adjacent recipient sheet to transfer electrically conductive printing material particles from the donor sheet to the recipient sheet. The shape of the resultant deposit of printing material particles on the recipient sheet is related to the spatial distribution of the pulsed electric field components passing through the donor and recipient sheets. The distribution of those components is primarily controlled by the shape of the printing face of the shaping electrode. A shield electrode is disposed about the outer contour of the printing face of the shaping electrode and extends along the side surfaces of that electrode for cancelling the effect of fringe components of the pulsed electric field emanating from points on the surface of the shaping electrode outside the printing face.

23 Claims, 5 Drawing Figures PATENTEU AUG 51975 SHEET GENERATOR IH IHH'HI Ham GENERATOR FIG-.4

BACKGROUND OF THE INVENTION This invention relates to printing methods. and more particularly to methods employing an electric field to deposit mobile printing particles from a donor sheet to a recipient sheet.

It is known in the art to produce high speed, nonimpact printing by applying a pulsed electric field between an electrode pair across a donor sheet and a closely adjacent recipient sheet. For example. see the U.S. Pat. to Haeberle et al, No. 3.550,l53. assigned to the assignee of the present invention. In accordance therewith, the surface of the donor sheet closest to the recipient sheet includes electrically conductive particles of a printing material dispersed in a high resistance medium. The pulsed electric field is applied to selectively charge the printing particles of the donor sheet. Those charged particles are subsequently transferred to the adjacent surface of the recipient sheet under the influence of the applied field.

The I-Iaeberle printer utilizes an efficient charging technique whereby a charge is imparted to the printing particles in a brief space of time, thereby providing for a substantial accelerating force on the particles and allowing a high speed printing process. This efficient charging technique is primarily enabled by the use of a high amplitude pulsed electric field, having a potential difference between electrodes that may range from several hundreds of volts to more than one thousand volts in various printer configurations.

The spatial distribution of the applied electric field as that field passes through the donor and recipient sheets controls the shaped image formed by the deposit of printing particles on the recipient sheet. As disclosed by Haeberle, the distribution of the electric field is primarily controlled by the shape of the tip, or printing face, of a shaping electrode which is disposed adjacent to the recipient sheet. The electric field components produced by that electrode are terminated by a base electrode, which base electrode is large compared with the shaping electrode. The base electrode lies adjacent to the donor sheet so that the pulsed field emanating from surface charges on the shaping electrode passes wholly through the donor and recipient sheets.

Printers operating in accordance with the Haeberle patent may be ofa form in which the field shaping electrode is an alphanumeric character, or in which a printing head comprises a plurality of shaping electrodes arranged in a matrix, wherein each electrode may, for example, have a circularly shaped printing face. In the matrix form, each shaping electrode is arranged so that selected ones of the matrix may be energized to produce a resultant sequence of dots (i.e. depositions of printing particles) in the shape of an alphanumeric character.

Such printing devices are, however, subject to a substantial disadvantage in that the shaped electric field emanating from the shaping electrode includes both a component from surface charges on the printing face ofthe shaping electrode and an undesirable fringe component from surface charges on the side surfaces of the shaping electrode. The effect of the fringe component is to increase the area of the donor and recipient sheets through which the field passes. The resultant dispersion of deposited printing particles due to the fringe compo nent of the pulsed electric field establishes substantial limits on the resolution of printing which may be accomplished using the printing technique as disclosed by Haeherle.

SUMMARY OF THE INVENTION Accordingly. it is an object of the invention to provide a new and improved method of high speed. nonimpact printing.

Another object ofthe invention is to provide a means for high speed. non-impact printing of high resolution characters.

In accordance with a present invention. the sharpness and clarity, and hence the resolution, of high speed, nonimpact printing under the Haeberle process is improved through the addition of a conducting shield electrode associated with the individual shaping electrodes in a printing head. The shield electrode is insulated from the shaping electrodes and, in the simplest embodiment, is maintained at the same ground reference potential as is the base electrode. The shield electrode is effective to terminate electric field lines emanating from surface charges on the side surfaces ofthe shaping electrode, and thus serves as a shield to limit the electric field passing through the donor and recipient sheets to be from the printing face of the shaping electrode. In a more complex embodiment having a suitable geometry, a pulse of an inhibitory character is applied to the shielding electrode. In such an embodiment the inhibitory pulse is of a lower amplitude than the printing pulse applied to the shaping electrode and of opposite polarity thereto.

The net effect of the shield electrode is to provide control over the spatial distribution ofthe residual electric field components in the region of the donor and recipient sheets where printing is desired. The residual pulsed electric field components which pass through the sheets emanate from the printing face of the shaping electrode. The spatial distribution of these residual field components in the printing region is substantially the same as the distribution of field components emanating from the printing face of the shaping electrode. with the amplitude of the field diminishing with a relatively steep gradient beyond the region where printing is desired. Consequently, a relatively sharp transition is produced between a printed region and a non-printed region, thereby providing a high resolution image.

Other features of the invention will be evident from the following description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects of this invention, the various features thereof, as well as the invention itself may be more fully understood from the following description, when read together with the accompanying drawing in which:

FIG. 1 shows in semi-schematic form a printing structure in accordance with the present invention; 7

FIG. 2 shows a sectional view of the structure of FIG.

FIG. 3 shows a cross-section of the electric field distribution of a printing structure in accordance with the prior art;

FIG. 4 shows a cross-section of the electric field distribution of the printing structure of FIG. 1; and

FIG. 5 shows a matrix printing structure in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A high speed. non-impact printer in accordance with the present invention is shown in FIGS. 1 and 2. In FIG. I, the electric field shaping electrode 12' and base electrode 14 are shown to be separated by a printing gap 15. In that gap are shown a donor sheet 17 and a recipient sheet 19. The relative dimensions of the printing structure in FIG. 1 are exaggerated compared to those of the sheets 17 and 19 for clarity. The surface of donor sheet 17, which is closest to recipient sheet 19. will be understood to include a plurality of electrically conductive particles of a printing material or pigment dispersed in a high resistance medium.

The dimensions of the printing structure and donor and recipient sheets 17 and 19, and their position within gap 15 are all selected so that the structure operates in accordance with the teachings of Haebcrlc, i.e. a pulsed electric field is applied between electrodes 12 and 14 across sheets 17 and19 to selectively charge printing particles dispersed in donor sheet 17.' The pulsed electric field is further effective to transfer those charged particles to the adjacentsurface of recipient -sheet 19. A visible image is thereby formed by the deposition of the charged printing particles on sheet 19. The electric field' is-produced by a pulse signal applied .to shaping electrode 12 from generator 21. Base electrode 14 is maintained at a gropnd reference potential.

Also shown in FIGS. 1 and2 is a cylindrical shield electrode 23, which electrode comprises an electrically conductive material and is disposed about the side surfaces of shaping electrode 12. Shield electrode 23 is separated by shielding gap 27 from shaping electrode 12 by meansof an electrically insulating 'material 25. As shown in FIG. 1, electrode 23 is maintained at a ground reference potential. In-the above'described ernbodimcnt, electrode 23 is supported together'wit h electrode 12 by material 25.

It .will' be understood that the shaping electrode 12 provides the principle, means of shaping the I image formed by the deposition of the printing particles on recipient sheet 19. In the configuration shown, that image will be circular, that is. substantially the same shape as the printing face of electrode 12, i.e., the surface portion of electrode 12 closest to donor sheet 17. In accordancewith the presentinvention, the shielding gap'27 between electrode 12 and electrode 23 is selected in relation to the dimension of gap.15 so that when the shield electrode 23 is maintained at an appropriate potential substantially all the fringe components of the electric field emanating from surface charges on the side surfaces of electrode 12 are terminated by surface charges on electrode 23, and virtually none of the fringe components emanating from side portions of electrode 12 passthr'ough sheets 17 and 19 to reach electrode 14. In this manner, a printing structure is provided in accordance with the present invention wherein the electric field components which reach electrode 14 are substantially only those emanating from the surface of electrode 12 which is closest to recipient sheet 19, i.e. the printing face. Those fringe field components emanating from the side surface ofelectrode 12 are terminated by the shield electrode 23. As a result. the field amplitude at sheet 19 diminishes with a relatively steep gradient beyond the region in which printing is desired.

This effect is depicted in FIGS. 3 and 4. FIG. 3 shows a prior art configuration as taught by Haeberle in the above referenced patent. and FIG. 4 shows a configuration in accordance with the present invention. In FIG. 3, the units and tens digits of the reference numerals relate the elements of the printing structure to the corresponding elements of the structure shown in FIG. 1. In FIG. 4, the reference numerals relate the components of the printing structure to those shown in FIG. 1.

In the Haeberle structure of FIG. 3, a single cylindrical shaping electrode 112 having radius R is shown with its printing face being separated from base electrode 114 by gap 115 (3R or three times the radius of electrode 112). Electrode 112 is driven by generator 121 in order to establish a pulsed electric field between electrode 112 and ground electrode 114. The electric field thereby produced is depicted by the broken lines of FIG. 3, wherein the density of the field lines at a point in FIG. 3 is indicative of the field amplitude at that point. (It will be noted that FIGS. 3 and 4 show the electric field pattern in an embodiment with the dielectric material 25 having the same dielectric constant as the gap region. In embodiments having differing dielec tric constants in these regions, the electric field has a discontinuity inits normal component at the boundary between the two regions). The field lines emanate at right angles from electrode 112, pass through recipient sheet 119 and donor sheet 117 and terminate at right angles to the top surface of electrode 114. The fringe field components from the side surfaces of electrode 112 start out parallel to the surface of the sheets 119 and 117 and then curve toward the donor and recipient sheets. It will be noted from the field line density shown in FIG. 3 that the sheets 117 and 119 are subjected to a maximum intensity field in the region closest to the center of the printing face of electrode 112. It is assumed for the hereindescribed embodiment that the amplitude of the applied electric field is selected so that for the printing structure shown and for donor and recipient sheets, as described by Haeberle, the conductive particles in donor sheet 117 are transferred to recipient sheet 119 to form a dark' region where the amplitude of the electric field passing through sheets 117 and 119 is between and l00% of the peak electric field, E passing from the center of the printing face of electrode 112, a gray" region where the amplitude of the electric field passing through sheets 117 and 119 is between 30 and 70% of the peak field E and a light" (or no printing) region where the field amplitude is below 30% of the peak field, E

The heavy shaded region 120a on the lower surface of sheet 119 represents the dark region of an image formed by the deposit of printing particles on that sheet as caused by the electric field (having intensity between 70 and of the peak) passing through the sheets 117 and 119. The cross-hatched region l20b represents the gray region of the image formed on sheet 119. The gray region is representative of the transition between a printed region and a non-printed region. It will be noted that the width of the full image including regions a and b is substantially greater than the width of the printing face of electrode 112. Further, it will be noted that the major portion of the extension of image beyond the limits of electrode 112 is clue to the fringe electric field components emanating from the side surfaces of electrode 112.

For comparison with FIG. 3, FIG. 4 shows a printing structure embodying the present invention and being otherwise the same as thestructure of FIG. 3. Specifically. a grounded annular shield electrode 23 (of thickness 2R) is disposed at a distance 2R from the shaping electrode 12 so that the ratio of the dimensions of the printing gap and the shield gap is proportional to the ratio 3/2. Generator 21 produces a pulsed signal having a magnitude as such that the peak field in gap is the same as the peak field in gap 115 produced by generator 121 in FIG. 3. The pulsed signal applied to electrode 12 produces an electric field between electrode 12 and electrode 14 as depicted by the field lines emanating from electrode 12. As in 'FIG. 3, the density of the depicted field lines at a point is indicative of the field amplitude at that point. For the structure shown in FIG. 4, the peak field passing through sheets 17 and 19 occurs at a point closest to the center of the printing face of the shaping electrode 12, as in the structure of FIG. 3. The fringe field lines emanating from the side portions of electrode 12 terminate on the shield electrode 23 which is maintained at the ground potential, and not on electrode 14 as are the corresponding fringe components shown in FIG. 3. As a result, only those field lines which emanate from surface charges on the circular printing face of electrode 12 reach the donor and recipient sheets and cause printing. 7

As in the structure of FIG. 3, a dark region is defined to include those regions where the field amplitude is between 70 and 100% of the peak amplitude, a

gray region where the field is between 30 and 70% of the peak amplitude, and a light region where the field is below 30% of the peak amplitude. In FIG. 4, the heavy shaded region a on sheet 19 represents the dark region and the cross-hatched area 20!) on sheet 19 represents the gray region. It will be noted that the width of the printing region of FIG. 4 (including regions 20a and 20b) is substantially smaller than the corresponding region for the structure of FIG. 3. These regions are primarily formed by those components of the pulsed electric field, which emanate from the surface of the electrode 12 which is closest to donor sheet 17, i.e., the printing face of that electrode. Since the fringe components emanating from the side of electrode 12 are terminated by the shield of electrode 23, none of those components are effective to cause a deposition of printing particles on recipient sheet 19.

In addition, the width of the gray region 20b of FIG. 4 is substantially smaller than the gray region 120 of FIG. 4. Consequently, the edge definition of the image produced by the present invention is correspondingly better than that of the prior art. Thus, a fine wire shaping electrode of suitable geometry with a shielding sleeve electrode will print a comparatively fine dot. For the exemplary configurations described above, this improvement in resolution is on the order of 3:2.

In an other embodiment having an alphanumeric character-shaped electrode. a shielding sleeve electrode following, but spaced from, the outer contour of the shaping electrode will similarly produce a high resolution character with sharp" edges. In still other embodiments, electrode 23 may be maintained at a different potential than ground potential which, for the geometry of the particular printing structure; provides control of the fringe field. Such control includes the capability of dispersing the field from the printing face to produce an expanded image in comparison to the print face. Alternatively. a pulse signal of opposite polarity to that applied to electrode 12 (and ofa predetermined amplitude) may be applied to electrode 23.

It will be understood that in both FIGS. 3 and 4 the electrodes 112, I2, and 23 arc'shown to be supported in a dielectric medium 25. In other embodiments other structures maybe used. By way of example. a printing structure may comprise a thin wire embedded in a glass capillary tube, wherein the outer surface of the tube has an electrically conductive coating.

An embodiment is shown in FIG. 5 wherein sixteen cylindrical field shaping electrodes 12 are disposed in a matrix. The shaping electrodes 32 are arranged in a conductive mounting block 35, wherein each electrode 12 is isolated from the shield (block 35) by a dielectric material 25. In other embodiments. different numbers of shaping electrodes may be used. In still other embodiments, a plurality of individually shielded electrode assemblies may be mounted in a dielectric block. Such an electrode assembly may comprise, for example, a fine wire centrally disposed in a glass capillary tube, wherein the tube outer surface is plated with a conductive coating. In the latter type arrangement. a separate inhibitory shielding pulse may be selectively applied to the individual electrode assemblies that are energized.

A matrix printing structure of form shown in FIG. 5 may be used for facsimile printing or multi-dot alphanumeric character printing. The resolution in such a printing head is limited by the envelope of the electric field components of the respective shaping electrodes as the field passes through the recipient and donor sheets.

In a matrix print head in the prior art where the shaping electrodes are fine wires, such as are used for facsimile printing, and no shielding sleeve is used (for example, where the shaping electrodes are imbeddcd in an insulating material), the fringe field limits the size of the printed dot to be at least four times the dimension of the gap region between the shaping and base electrodes, irrespective of the size of the individual shaping electrode printing faces. In all practical systems. this gap is already constrained to be of a substantial size, for example 0.006 inches, since the gap must contain the donor and recipient sheets with the necessary clearance space. Thus, the minimum radius for a printed dot produced by a single one of the shaping electrodes in the matrix is limited since the fringing fields spread the actual print to a considerably larger radius. As a result. in the design of a prior art matrix printing head. the spatial density of the shaping electrodes is severely limited.

In the configuration of FIG. 5, embodying the present invention, a pulse generator 21 is connected via character selection logic 41 to the various ones of shaping electrodes 12. Logic 41 may have any form known in the art which is effective to gate the pulse signal from generator 21 to the appropriate ones of electrodes 12 to energize those electrodes so that a predetermined alphanumeric character is printed on sheet 19. Generator 21 and logic 41 may be of any suitable form known in the art and are not considered a part of this invention.

In the above configuration, with a gap dimension of 0.006 inches, fine wire shaping electrode of diameter 0.004 inches, and a cylindrical shield electrode spaced 0.004 inches from the shaping electrode, a dark dot having a 0.0064 inch diameter with 110.0028 inch gray area may be printed with a 800 volt. 5 microsecond pulse. A similar sized dark dot printed with a printing structure of the prior art having an identical peak field amplitude but having no shield electrode has a 0.0043 inch gray area. Thus. a printing head in accordance with the present invention provides an improvement in edge definition and resolution on the order of507( over a comparable device of the prior art.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive. the scope of the invention being indicated by the appended claims rather than by the foregoing description. and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

I claim:

1. In the method of printing an image on a recipient sheet which method includes the step of applying a pulsed electric field through said recipient sheet and an adjacent donor sheet. said donor sheet having mobile electrically conductive printing particles dispersed in a high resistance medium on the surface of said donor sheet which is adjacent to said recipient sheet. said field being applied between a shaping electrode and a base electrode. said shaping electrode including a printing surface and a side surface extending therefrom, said printing surface having a shape corresponding to the image to be printed. wherein the improvement comprises the further step of:

limiting the spatial distribution of said applied electric field with a shield electrode by maintaining said shield electrode at a predetermined electric potential. insulating said shield electrode from and disposing it about the side surface of said shaping electrode with a portion of said shield electrode surrounding the outer contour of said printing surface. said portion being separated therefrom by a shielding gap. maintaining said shielding gap to be uniform such that all points on said portion are substantially equidistant from said outer contour. and establishing said shielding gap to have a magnitude related to the magnitude of the gap between said shaping and base electrodes and to said electric potential of said shield electrode so that substantially all the electric field components emanating from the side surfaces of said shaping electrode terminate on said shield electrode.

2. In a printing means employing an electric field to deposit mobile printing particles from a donor sheet to a recipient sheet, said printing means having a field shaping electrode with a printing surface and a side surface extending therefrom. and having a base electrode disposed parallel to and separated by a printing gap from said printing surface of said shaping electrode. wherein the improvement comprises:

a shield electrode electrically insulated from and disposed about said side surface of said shaping electrode and having a portion surrounding the outer contour of said printing surface. said portion being separated from said printing surface by a uniform shielding gap such that all points on said portion are substantially equidistant from said outer contour. the magnitude of said shielding gap being related to the magnitude of said printing gap so that substantially all the electric field components emanating from said side surfaces of said shaping elec trode terminate on said shield electrode when said shield electrode is maintained at a predetermined electric potential relative to said base electrode.

3 A printing means in accordance with claim 2, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

4. A printing means according to claim 2 having a support for said shaping and shield electrodes. comprising a tube-like structure surrounding the shaping electrode.

5. A printingmeans in accordance with claim 2, including a means for applying a first pulsed electric field between said shaping electrode and said base electrode.

6. A printing means in accordance with claim 5, in' cluding a means for maintaining said shield electrode at an electric potential to prevent electric field components emanating from said side surface of said shaping electrode from being applied through said donor and recipient sheet. and to allow electric field components from said printing surface to pass therethrough.

7. A printing means in accordance with claim 6. wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

8. A printing means in accordance with claim 5, including a means for applying a second pulsed electric field between said shield electrode and said base electrode. said second pulsed electric field being of opposite polarity to said first pulsed electric field and having a magnitude to prevent electric field components emanating from said side surface of said shaping electrode from being applied through said donor and recipient sheets. and to allow electric field components emanating from said printing surface to pass therethrough.

9. A printing means in accordance with claim 8, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

10. In a printing means employing an electric field to deposit mobile printing particles from a donor sheet to a recipient sheet. said printing means having a plurality of field shaping electrodes arranged in a matrix. each of said shaping electrodes having a printing surface and a side surface extending therefrom. and said printing means having a base electrode disposed parallel to and separated by a printing gap from said printing surfaces of each of said plurality of shaping electrodes. wherein the improvement comprises:

a plurality of shield electrodes. each of said shield electrodes being associated with one of said shaping electrodes and being electrically insulated from and disposed about the side surface of its associated shaping electrode and having a portion surrounding the outer contour of its associated printing surface. each of said portions being separated from the printing surface of its associated shaping electrode by a uniform shielding gap such that all points on said portions are substantially equidistant from the outer contour of the printing surface ofits associated shaping electrode. the magnitude of said shielding gap being related to the magnitude of said i printing gap so that substantially all the electric field components emanating from the side surfaces of each of said shaping electrodes terminate on the associated shield electrode when said associated shield electrode is maintained at a predetermined electric potential relative to said base electrode.

11. A printing means in accordance with claim 10. wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

12. A printing means in accordance with claim 10., including a mean for selectively applying a first pulsed electric field between at least one of said shaping electrodes and said base electrode.

13. A printing means in accordance with claim 12, including a means for maintaining said shield electrodes associated with said selected shaping electrodes having said electric field applied thereto at an electric potential to prevent electric field components emanating from said side surface of said selected shaping electrodes from being applied through said donor and recipient sheets. and to allow electric field components from said printing surfaces to pass therethrough.

14. A printing means in accordance with claim 13, including a means for maintaining each of said shielding electrodes at the same electric potential.

15. A printing means in accordance with claim 14, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

16. A printing means in accordance with claim 13, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

17. A printing means according to claim 13 wherein said each shaping electrode is coaxially disposed within an associated insulating capillary tube having an outer radial dimension equal to said shielding gap.

18. A printing means according to claim 17 wherein said capillary tubes are supported in said matrix arrangement by a conductive medium.

19. A printing means according to claim 17 wherein the shield electrode associated with each field shaping electrode is disposed on the outer surface of the associated capillary tube.

20. A printing means in accordance with claim 12. including a means for selectively applying a second pulsed electric field between said base electrode and the ones of said shield electrodes associated with said selected shaping electrode having said first electric field applied thereto said second pulsed electric field being of opposite polarity to said first pulsed electric field and having a magnitude to prevent electric field components emanating from said side surfaces of said selected shaping electrodes from being applied through said donor and recipient sheets. and to allow electric field components emanating from said printing surfaces to pass thercthrough.

21. A printing means in accordance with claim 20. wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.

22. A printing means in accordance with claim 20. including a means for maintaining each of said shielding electrodes at the same electric potential.

23. A printing means in accordance with claim 22, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2. 

1. In the method of printing an image on a recipient sheet which method includes the step of applying a pulsed electric field through said recipient sheet and an adjacent donor sheet, said donor sheet having mobile electrically conductive printing particles dispersed in a high resistance medium on the surface of said donor sheet which is adjacent to said recipient sheet, said field being applied between a shaping electrode and a base electrode, said shaping electrode including A printing surface and a side surface extending therefrom, said printing surface having a shape corresponding to the image to be printed, wherein the improvement comprises the further step of: limiting the spatial distribution of said applied electric field with a shield electrode by maintaining said shield electrode at a predetermined electric potential, insulating said shield electrode from and disposing it about the side surface of said shaping electrode with a portion of said shield electrode surrounding the outer contour of said printing surface, said portion being separated therefrom by a shielding gap, maintaining said shielding gap to be uniform such that all points on said portion are substantially equidistant from said outer contour, and establishing said shielding gap to have a magnitude related to the magnitude of the gap between said shaping and base electrodes and to said electric potential of said shield electrode so that substantially all the electric field components emanating from the side surfaces of said shaping electrode terminate on said shield electrode.
 2. In a printing means employing an electric field to deposit mobile printing particles from a donor sheet to a recipient sheet, said printing means having a field shaping electrode with a printing surface and a side surface extending therefrom, and having a base electrode disposed parallel to and separated by a printing gap from said printing surface of said shaping electrode, wherein the improvement comprises: a shield electrode electrically insulated from and disposed about said side surface of said shaping electrode and having a portion surrounding the outer contour of said printing surface, said portion being separated from said printing surface by a uniform shielding gap such that all points on said portion are substantially equidistant from said outer contour, the magnitude of said shielding gap being related to the magnitude of said printing gap so that substantially all the electric field components emanating from said side surfaces of said shaping electrode terminate on said shield electrode when said shield electrode is maintained at a predetermined electric potential relative to said base electrode.
 3. A printing means in accordance with claim 2, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.
 4. A printing means according to claim 2 having a support for said shaping and shield electrodes, comprising a tube-like structure surrounding the shaping electrode.
 5. A printing means in accordance with claim 2, including a means for applying a first pulsed electric field between said shaping electrode and said base electrode.
 6. A printing means in accordance with claim 5, including a means for maintaining said shield electrode at an electric potential to prevent electric field components emanating from said side surface of said shaping electrode from being applied through said donor and recipient sheet, and to allow electric field components from said printing surface to pass therethrough.
 7. A printing means in accordance with claim 6, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.
 8. A printing means in accordance with claim 5, including a means for applying a second pulsed electric field between said shield electrode and said base electrode, said second pulsed electric field being of opposite polarity to said first pulsed electric field and having a magnitude to prevent electric field components emanating from said side surface of said shaping electrode from being applied through said donor and recipient sheets, and to allow electric field components emanating from said printing surface to pass therethrough.
 9. A printing means in accordance with claim 8, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.
 10. In a printing means employing an electric field to deposit mobile printing particles from a donor sheet to a recipient sheet, said printing means having a plurality of field shaping electrodes arranged in a matrix, each of said shaping electrodes having a printing surface and a side surface extending therefrom, and said printing means having a base electrode disposed parallel to and separated by a printing gap from said printing surfaces of each of said plurality of shaping electrodes, wherein the improvement comprises: a plurality of shield electrodes, each of said shield electrodes being associated with one of said shaping electrodes and being electrically insulated from and disposed about the side surface of its associated shaping electrode and having a portion surrounding the outer contour of its associated printing surface, each of said portions being separated from the printing surface of its associated shaping electrode by a uniform shielding gap such that all points on said portions are substantially equidistant from the outer contour of the printing surface of its associated shaping electrode, the magnitude of said shielding gap being related to the magnitude of said printing gap so that substantially all the electric field components emanating from the side surfaces of each of said shaping electrodes terminate on the associated shield electrode when said associated shield electrode is maintained at a predetermined electric potential relative to said base electrode.
 11. A printing means in accordance with claim 10, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.
 12. A printing means in accordance with claim 10, including a mean for selectively applying a first pulsed electric field between at least one of said shaping electrodes and said base electrode.
 13. A printing means in accordance with claim 12, including a means for maintaining said shield electrodes associated with said selected shaping electrodes having said electric field applied thereto at an electric potential to prevent electric field components emanating from said side surface of said selected shaping electrodes from being applied through said donor and recipient sheets, and to allow electric field components from said printing surfaces to pass therethrough.
 14. A printing means in accordance with claim 13, including a means for maintaining each of said shielding electrodes at the same electric potential.
 15. A printing means in accordance with claim 14, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.
 16. A printing means in accordance with claim 13, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2.
 17. A printing means according to claim 13 wherein said each shaping electrode is coaxially disposed within an associated insulating capillary tube having an outer radial dimension equal to said shielding gap.
 18. A printing means according to claim 17 wherein said capillary tubes are supported in said matrix arrangement by a conductive medium.
 19. A printing means according to claim 17 wherein the shield electrode associated with each field shaping electrode is disposed on the outer surface of the associated capillary tube.
 20. A printing means in accordance with claim 12, including a means for selectively applying a second pulsed electric field between said base electrode and the ones of said shield electrodes associated with said selected shaping electrode having said first electric field applied thereto, said second pulsed electric field being of opposite polarity to said first pulsed electric field and having a magnitude to prevent electric field components emanating from said side surfaces of said selected shaping electrodes from being applied through said donor and recipient sheets, and to allow electric field components emanating from said printing surfaces to pass therethrough.
 21. A printing means in accordance with claim 20, wherein the ratio of said printing gap to said shielding gap is proportIonal to the ratio 3/2.
 22. A printing means in accordance with claim 20, including a means for maintaining each of said shielding electrodes at the same electric potential.
 23. A printing means in accordance with claim 22, wherein the ratio of said printing gap to said shielding gap is proportional to the ratio 3/2. 